Electro-mechanical transducer



June 10,l 1969 W- J- ASHWORTH 3,449,531

ELECTRO-MECHANICAL TRANSDUCER Filed Jan. 9, 1968 Shee'fI FIG.

FIG.5

F I G. 7

June 10, 1969 w. J. Asl-:WORTH ELECTRO-MECHANICAL TRANSDUCER Filed Jan.9, 1968 Sheet .2 of 2 .93 56 6 7 VAWNNNNNNWA e .4

4 slwe O 6 A M3 6 6 `W s: 2 6 4 5 u 4 6 FIG. 9

FIG. 8

FIG.

INVENTOR wmgwm FIG.

United States Patent O 3,449,531 ELECTRO-MECHANICAL TRANSDUCER WilliamJ. Ashworth, Rte. 2, New Albany, Miss. 38652 Filed Jan. 9, 1968, Ser.No. 696,632 Int. Cl. H04r 7/02 U.S. Cl. 179--115 9 Claims ABSTRACT F THEDISCLOSURE A sound system using a transducer or sound reproducer, havinga radiating exible diaphragm which is attached to a sounding board, forexample, walls, doors, car body, etc., causing the sounding board toradiate sound. The electromagnet driving member is resiliently attachedto the armature so that the armature and driving members are capable ofrelative, interacting motion and are thereby capable of good lowfrequency response as well as a good mid range and high frequencyresponse. The resilient mounting can be accomplished by either springsor resilient pads. Besides sound reproduction, other uses for the soundreproducer are available.

Background of invention This application is a continuation in part of mycopending application, Ser. No. 541,209, filed April 8, 1966 andentitled Combination Momentum and Inertia Sound Reproducer, nowabandoned.

The present invention relates to a magnetically activated transducer,useful particularly in the production of sound and is particularlydesigned as a transducer unit to be attached to a sound radiating sourceor sounding board such as a wall, door, or car body, for example.

Heretofore, other electrical sound reproducing devices of the samegeneral type as that described above, have been proposed, for example,that shown in my prior U.S. Patent 3,178,512, issued Apr. 13, 1965, andmy U.S. Patent 3,334,195, issued Aug` l, 1967 but will have had responseof only moderate fidelity.

Summary of invention The present invention solves the low frequencyresponse problem by a unique resilient suspension system whereby theelectromagnet driving member and the armature member are capable ofrelative, interacting motion and permit mutual power transfer betweenthe two. Many reiinements and improvements utilizing the uniquetransducer of the present invention have been devised and are disclosedand claimed below.

Other objects and advantages of the invention will be apparent from thefollowing description taken in conjunction with the accompanyingdrawings.

Description of the drawings FIG. 1 is an end elevation view of oneembodiment of the present invention.

FIG. 2 is a side elevation view of another embodiment of the presentinvention.

FIG. 3 is a front elevation of a further embodiment of the presentinvention.

FIG. 4 is an elevation view of a still further embodiment of the presentinvention.

FIG. 5 is an elevation view showing an alternate construction of thepresent invention.

FIG. 6 is a sectional view taken along line x--x of FIG. 5.

FIG. 7 is an elevation view of the present invention showing a furtheralternate construction.

FIG. 8 shows a method of using the present invention n combination witha loudspeaker.

3,449,531 Patented June 10, 1969 ICC FIG. 9 shows another method ofusing the present invention in combination with a loudspeaker. FIG. 10shows a further method of using the present lnvention in combinationwith a loudspeaker.

FIG. 11 shows a still further method of using the present invention incombination with a loudspeaker.

Description of the preferred embodiments An electromagnet 10 is mountedon the lower side of upper plate 13, substantially centered with plate13 and comprises a laminated E shaped frame or core 11 having threeequally spaced arms or poles and on the center arm or pole 11 of whichis mounted an electrical winding, comprising a coil 12, whereby theelectromagnet may be energized when the coil leads 51 are connected to asignal source.

The diaphragm 15 is non-rigidly or flexibly attached to the upper plate13 in the following manner. The four screws 19 extend through fourequally spaced apertures in the upper plate 13 parallel with the legs ofthe E frame core 11 and the lower ends of the screws 19 are connected tothe upper ends of four coiled spring 18 made of brass. The opposite endsof the four brass springs 18 are firmly connected to the upper face ofthe diaphragm 15. As shown, the four screws 19 are longitudinallyadjustable in the four apertures of plate 13 by means of locknuts 20threaded onto the screws 19 one on each side of the upper plate 13.

The function of the adjustable screws 19 are to adjust the spacingbetween the diaphragm 15 and the pole pieces of the electromagnet. Thisspacing is critical for optimum performance. The closer the armature ispositioned to the electromagnet pole pieces the greater is theefliciency of operation. However, if the armature spacing is too closeto the electromagnet pole pieces, metal to metal contact between thearmature and the pole pieces will result.

On the other hand, if the armature is spaced too far away from theelectromagnet pole pieces, the eilciency of the sound reproduced will beconsiderably lowered. It is extremely ditiicult to maintain this optimumspacing under production conditions. A number of problems complicatesthe task of maintaining the critical spacing between the electromagnetpole pieces and the armature. If a production parts should slightlyvary, the spacing would be incorrect. Tension in various parts couldvary and this would require a different spacing between the armature andthe electromagnet pole pieces. To overcome the problem of maintainingthis critical spacing the system of a1- lowing the diaphragm 15 to beadjusted in or out with the locknuts and the screws was devised. Asshown above, the screws 19 are rigidly fastened to brass springs 18 withnuts 21 and are in turn welded to the diaphragm 15. They are supportedby the upper plate 13 and held rigidly in place with locknuts 20.Diaphragm 15 is pulled toward core 11 by the pull of the electromagnetand is held in a spaced relationship with the electromagnet core 11 bysprings 18. This spaced apart relationship can be adjusted in or out byadjusting locknuts 20 until the opti-mum spacing between the diaphragm15 and the electromagnet core 11 is obtained. This eliminates thenecessity of attempting to maintain very close fixed tolerances inproduction. This adjustment is especially advantageous to the presentinvention because of the relatively long movement between the diaphragm15 and the electromagnet at certain operating frequencies. When thediaphragm 15 of the transducer embodying the present invention issolidly attached to a sounding board 16 such as a door, wall, ceiling,or other object and an electrical signal is fed into lead wires 51 ofthe electromagnet the transducer will cause the sounding board 16 tovibrate and radiate sound.

At this point it will be realized that as the electromagnet is activatedby a variable electrical current passing through the coil 12, the pole11 will become Variably magnetized and so effect a variable magneticattraction on the diaphragm 1S thereby imparting vibrations to the soundboard 16 onto which the diaphragm 15 is attached.

The brass springs 18 also serve as a resilient restoring means for thenormal inactive position of the diaphragm 15 as well as serving as aconnector for the spacing means between diaphragm 15 and theelectromagnet poles.

An alternate construction of the present invention is shown in FIG. 3.The difference in this construction and the construction in FIG. 1 is inthe spacing allowed between the diaphragm 15 and the sounding board 16.The diaphragm 15 and mounting plate 27 are held in a spaced apartrelationship by the four equally spaced spacers at the outer edge of thediaphragm 15. When these two plates are fastened together and spacedapart by the spacers 28 they become the dual diaphragm armatureassembly. This dual diaphragm armature assembly is connected to theupper plate 13 in the same way and by the same means as the diaphragm 15in FIG. 1.

A modification of the alternate construction shown in FIG. 3 is shown inFIG. 2 and accomplishes the same result as the dual diaphragm armatureassembly as shown in FIG. 3. As shown, an attaching screw 17 iscentrally welded to the lower diaphragm 15. Screw 17 serves as anattaching means for solidly attaching the transducer to a soundingboard. The head of screw 17 acts as a spacer to hold diaphragm 15 in aspaced relation with sounding board 16.

Due to the weight and inertia of the magnetic core, the coil 12 and themounting hardware, and because of the flexible resilient mounting means18, enough weight is present to cause the electromagnet portion of thetransducer to remain relatively stable and free from oscillations atoperating frequencies above approximately 150 cycles per second. Whenoperating above 150 cycles through approximately 2000 cycles per second,the diaphragm 15 oscillates on its resilient mounting, driving thesounding board structure it is attached to, causing sounding boardprimary radiating element 16 to vibrate and radiate sound.'

At operating frequencies below approximately 150 cycles per second,inertia no longer holds the mass of the electromagnet structure in afixed position and at these low frequencies the electromagnet is free tooscillate at a large amplitude with the relatively rigid or relativelyheavy sounding board 16 being a restraining medium. Because of thisrestraining action these intense oscillations of the electromagnetinteract with the diaphragm 15 and are transferred to the sounding board16. These intense oscillations are allowed because no restraining`attachments are connected with the electromagnet assembly other thandiaphragm through the flexible mounting 18. The nearer the frequency ofthe activating electrical signal approaches the natural mechanical lowresonant frequency of the transducer mass, the more intense thevibrations become. It is because of these intense vibrations that themomentum of the oscillating transducer mass will transfer enough drivingforce to the sound radiator 16 as to cause the sound radiator toreproduce good quality bass tones. In the past it has been attempted tocause large heavy surfaces to radiate sound like the described soundingboard radiating surface but to obtain any degree of bass response,extremely expensive and large equipment was utilized but with littlesuccess because of the natural resistance of a surface to radiate soundwith a lower tonal response than its own natural resonant frequency. Thepresent invention has solved this problem by causing extremely intensevibrations to -be imparted to the sound radiating surface 16 in thelower frequency ranges.

In order to reproduce good high frequency tones the constructions shownin FIG. 2 and 3 are utilized. At operating frequencies above`approximately 2000 cycles, the sounding board 16 has difficulty inreproducing these frequencies. At these frequencies, diaphragm 15, whenmounted in a spaced relation with sounding board 16 still remains activeand continues to radiate a high level of sound to approximately 8000cycles. The normal spacing between the diaphragm 15 and sounding board16 used in the present invention was approximately 1/16 inch althoughthis spacing is not critical.

lAs can be seen from the above description, three different functionscan be performed by the present invention, the first being to reproducethe mid range frequencies by one means, the second being to reproducethe low frequencies by a second means, the third being to reproduce thehigh frequencies by a third means. These three means are embodiedtogether in the structure of the present invention.

The present invention will operate either as a low impedance device oras a high impedance device. When used as a high impedance device, theconstant D.C. currant present in the activating signal is used toproduce a biasing magnetic field. The coil lead wires 51 are connecteddirectly to the plate of an audio amplifier output tube and B+. Such acircuit is not described in this disclosure because it is well known tothe art. A constant D.C. currant will flow through coil 12, setting up aconstant magnetic field around E frame core. This will in turn pull thediaphragm 15 toward the electromagnet, causing tension in springs 18.When a signal is applied to the output tube of the amplifier, thissignal will be superimposed on the direct current that is normallyflowing. The varying intensity of the signal will cause the transducerto function as previously described. The magnetic bias is necessary tothe proper operation of the transducer. This magnetic bias attracts thediaphragm to the electromagnet. The support screws 19 and springs 18serve as resilient adjustable spacing means to push the diaphragm awayfrom the electromagnet to provide for the proper air gap between theelectromagnet and the diaphragm 15.

In the transducer embodiments, the screws 19 are adjusted to provide agap of approximately 1/16 inch between the electromagnet pole pieces andthe armature assembly.

The diaphragm 15 is preferably a disc made of No. 24 gauge M19electrical steel, two inches in diameter. The upper plate 13 is a discmade of No. 20 guage soft iron, two inches in diameter. Each of theabove disclosed embodiments of the present invention comprises core ofan electromagnet made from standard 1/2 inch 29 guage M19 electricalsteel E lamination 11 stacked 1/2 inch high. A high impedance coil woundwith 4000 turns of No. 38 magnet wire may be used or a low impedancecoil of 200 turns of No. 25 wire may `be used. Coiled springs 18 aremade of 3 turns of .051 diameter spring brass wire wound in a 5/16 inchoutside diameter coil 1/2 inch long.

When it is desired to operate the present invention connected to a stepdown audio output transformer or to a transistor amplifier where noconstant DC current ow will be present, it is necessary to provide themagnetic bias with a permanent magnet added to the electromagnetstructure, for example, as in the embodiments shown in FIG. 4 and 5.

It should also be pointed out that the number of turns of wire requiredfor coil 12 will vary for the different impedance ratings of the variouskinds of driving equipment the transducer might be coupled to. Forexample, 200 turns of No. 25 wire wound on a `1/2 inch core of M19electrical steel laminations will have an inductive impedance ofapproximately 8 ohms at 400 cycles. For high impedance operation, 4000turns of No. 38 wire wound on a il/z inch core of M19 laminations willhave an inductive impedance of approximately 5000 ohms at 400 cycles.

Another embodiment of the persent invention is shown in FIG. 3. Where ahigh impedance primary winding 12 and a loW impedance secondary winding14 is wound on the core 11. The secondary -winding can be used toconnect directly to a conventional tweeter dynamic loudspeaker asdescribed and claimed in my Patent No.

E 3,178,512.7Coil 12 shown in FIGS. 1, 2 and 3 can be either a high or alow impedance winding. If a low impedance winding is used, a permanentmagnet must be disposed in the magnetic circuit of the electromagnet asdescribed and claimed in my Patent No. 3,358,084.

Different objects and surfaces will reproduce different qualities ofsound. The thinner and lighter the material to which the soundreproducer is mounted, the greater will be the volume of radiated soundfor a given amount of a signal power. A hollow core door covered with l;inch plywood is easier to drive and requires less driving power than apanel of 3A; inch sheetrock, therefore, the hollow core door will givelouder sound than the sheetrock if the same amount of power is appliedto the sound reproducer. The same will be true for 1A: inch plywoodpaneling and 3A inch knotty pine paneling. The lighter and thinner 1Ainch plywood ywill produce a greater volume of sound with the sameamount of power applied to the sound reproducer than will the inchknotty pine paneling.

The sound radiating material to which the sound reproducer is fastenedwill govern the sound quality obtainable. The sound reproducer willoperate well, however, with most materials except concrete and masonry.The sound reproducer can be attached to the sounding board with thescrew 17 in the center of the diaphragm 15 as shown in FIG. 2. Thediaphragm 15, FIG. 1, or attaching plate 27 of FIG. 3, may be cementeddirectly to the sounding board with a suitable cement. A spacing willexist between the diaphragm 15 and the object the sound reproducer isfastened to when the screw 17 is used as the attaching means as shown inFIG. 2. A head on the attaching screw may provide for this spacing.

Another embodiment of the present invention illustrated by FIG. 4 showsa low impedance winding 23 wound on core 22. Two permanent magnets 25are secured to the upper plate 13 with, for example, epoxy cement. Themagnets are magnetized longitudinally and are mounted so that the northpoles of the magnets are positioned in the same direction andmagnetically bias the electromagnet core 22 and diaphragm 15. Thediaphragm 15 is nonrigidly or flexibly attached to the upper plate 13.Additional resilient support means may be used by securing sponge rubberpads 29 between the ends of the magnets 25 and the diaphragm 15. Thesesponge rubber pads 29 dampen the intense oscillation of the transducerwhen the input signal to the transducer coincides with the mechanicalresonant frequency of the transducer. The sponge rubber resilient pads29 may :be optional in this embodiment of the present invention.

Core 22 is made from 29 gauge M19 electrical steel 1A: inch wide and 1%6inches long and the electromagnet core 22 is suitably mounted to theupper plate 13 as by a suitable cement such as epoxy cement. Thepermanent magnets 25 are made of sintered oriented barium ferrite andare 1 inch long, inch wide and We inch thick.

Another form of the present invention is shown in FIGS. 5, 6 and 7wherein a core 22 is centrally bonded to an iron cover plate or cap 31.A Winding 23 is wound on the core 22. A ring magnet 30 is bounded with asuitable cement to the inner side of the cover plate 31 which tits overthe electromagnet core 22 and coil 23.

A resilient foam rubber ring cushion 29', 3/16 inch thick, is cementedon the lower end of ring magnet 30 and diaphragm 15 is cemented to thecushion 29. Contact cement was found to be suitable for cementing theresilient cushion 29 to the magnet 30 and diaphragm 15.

For adjusting the air gap between the electromagnet core 30 and thediaphragm 15, the upper cover plate 31 has three equally spaced runners41 welded or formed on its underside which ride on slanting face-camsurfaces 34 molded in the top of ring magnet 30. As shown, the camsurfaces 34 are depressed 716 inch inward at their ends 35, 37, and 39and gradually rise to the level of the upper end of magnet 30 at theirends 36, 38, and 40, as

shown in FIG. 6 which is a view taken on line x--x of FIG. 5. When thecircular cover plate 31 is placed in position over ring magnet 30 therunners 41 rest on the cam surfaces 34. As can be seen from FIG. 6, ifthe cover plate 31 is turned clockwise with magnet 30 remainingstationary, the runners 41 Will follow the inward slope of the camsurfaces 34 lowering the cover plate in relation to magnet 30. Becausethe electromagnet core 22 is bonded to the cover plate it will also moveinward as the cover plate is turned clockwise. On the other hand, if thecover plate is turned in the opposite direction, the electromagnet core22 will move in an outward direction relative to the magnet 30.

It will now be apparent that by rotating the cover plate 31, the air gapbetween the core 22 and diaphragm can easily be adjusted for the correctspacing. When the proper spacing has been made, the cover plate 31 isthen locked in position with screw 32 which extends through the inclinedslot 33 in the sidewall of the cover 31 and screws into a threaded holein magnet 30.

It was found that by moving the steel cover plate 31 which provides themagnetic flux path between the core 22 and magnet 30, away from completesolid contact with the magnet 30 did not affect the performance of thetransducer.

Ring magnet 30 was made from sintered non-oriented barium ferrite, linch high with an outside diameter of 2 inches and an inside diameter of1 inch.

Another form of the present invention as shown in FIG. 7, has a basicconstruction similar to that as described for the construction in FIG.5. In this form a secondary coil 47 is added, consisting of 1,500 turnsof No. 38 wire and wound on the electromagnet core 22, which drives anelectrostatic operated piezeo element 46. This construction is andclaimed in my U.S. Patent 3,358,084. The piezeo element 46 is cementedto the underside of circular diaphragm 45 made of aluminum foil .008inches thick. Then the piezeo element 46 is activated by the secondarywinding 47, it in turn activates the foil diaphragm 45 producing a highlevel of sound in the high frequency range. Suitable frequency blockingor crossover networks may be used for selecting the frequency rangedesired for this tweeter section. Suitable networks are described in myU.S. Patent 3,358,084.

For better clarity, the leads from the secondary winding 47 are notshown in FIG. 7 but they are connected to each side of element 47.

As shown in FIG. 7, a spacer ring 49 is cemented on an upper circularplate 43 which carries the runners 41 and engages the cam surfaces 34.The foil diaphragm 45 is cemented to =the upper side of the spacer ring49 and a second spacer ring 48 is cemented to the upper side of foildiaphragm. A 2 inch circular piece of screen wire 44 is then cemented tothe upper side of spacer ring 48 and the entire assembly, comprisingparts 43, 44, 45, 46, 48, and 49, is cemented to the cover 42. Byturning cover 42, `the air gap is adjusted between the core 22 anddiaphragm as described for the construction shown in FIG. 5.

The construction shown in FIG. 7 utilizes a magnetizable metal surfaceas a sounding board. Foam rubber cushions 29 cemented to the lower sideof ring magnet 30 are placed in contact with the metal sounding board50. The magnetic attraction between the metal surface 50 and ring magnet30 will now hold the transducer to the stuface 50 without any otherholding means. This type of construction is especially useful when usedas u sound reproducing means for automobiles. The foam rubber pads 29may be cemented to sounding board 50 -When a permanent installation isdesired.

A transistor or other type of tiny radio receiver may be built insidethe transducer cover. The transducer is then attached to a soundingboard and a far better quality of sound can be obtained than can be hadwith a small loudspeaker, yet the entire mechanism is extremely small.

Another form of my invention is shown in FIGS. 8, 9, l0, and l1. Thisform is a continuation-in-part of my now copending application, Ser. No.558,204, tiled June 13, 1966 now abandoned.

The transducer 54 of the present invention and a conventional dynamicloudspeaker S5 are used in combination to form a unique sound system.The advantage of such a sound system resides in the fact that whentransducer 54 and loudspeaker 55 are driven by the same Signal, thesounding board 16 activated by transducer 54 and the paper diaphragm ofloudspeaker 55 have a difierent decap time. For example, when a giventone is fed to transducer 54 and loudspeaker 55 from amplifier 56 andsignal source 57, sounding board 16 and the diaphragm of speaker 55 willbe caused to vibrate. When the signal ceases, the vibration decay timeof the loudspeaker paper cone will be faster than the decay time of thelarge sounding board 16. This gives a very pleasing sound sensation tothe listener resembling true reverberation. This reverberation effectgives much more realism and is more pleasant to listen to than systemsutilizing the spring type units.

There is considerably less distortion present in the system using thetransducer of the present invention than in the reverberation systemsusing the spring type reverberation units such as described in -my U.S.Patent 3,174,- 121.

FIG. 8 shows transducer 54 and a loudspeaker 55 in parallel driven byamplifier 56 actuated by a signal source 57 which may be a radio tuner,turntable, or other signal source. Leads 62 connect the signal sourcewith the ampliter and leads 63 and 64 connect the amplifier output withthe sound reproducing means 55 and 54 respectively.

FIG. 9 shows transducer 54 and loudspeaker 55 connected in paralleldriven by amplifier 56 activated by signal source 57. Transducer 54 andloudspeaker 55 have a volume control 59 and 60 in their respective linesfor controlling the volume of each independently of the other.

FIG. 10 shows transducer 54 and loudspeaker 55 driven by individualamplifiers 56. This allows for individual control for the transducer 54and loudspeaker 55. Amplifiers 56 may be activated by the same signal oreach amplifier 56 may be activated by a stereo signal source 58 such asa stereo phonograph record.

FIG. l1 shows loudspeaker 55 driven directly through leads 63 byamplifier 56 with amplifier 56 being activated by signal source 57through leads 62. The input of amplifier 61 is connected to the outputof amplifier 56 by leads 65 and the output of amplifier 61 is connectedto transducer 54 by leads 64.

It should be understood that the transducer of the present inventionrequires more driving power to drive a large sounding board than isrequired to `drive a light loudspeaker paper cone. This is the reasonmeans are shown in FIGS. 9, 10 and l1 to regulate the power to eachsound reproducing means 54 and 55. The sound from each reproducing meansmay be balanced to the exact loudness best suited to the individuallistener.

Although several specific embodiments of this invention have been hereinshown and described, it will be understood that details of theconstructions shown may be altered or omitted without departing from thespirit of the invenion as defined by the following claims.

I claim:

1. An electro-mechanical transducer comprising a driving elementincluding a magnetic core and a coil of wire Awound on said core, saidcore being solidly attached to a mounting plate, an armature ofmagnetizable material mounted in axially spaced relation with saidmagnetic core, resilient coupling means connecting said `armatureelement with said mounting plate for permitting relative motion andmutual power transfer in all directions between said driving element andsaid armature element, magnetic biasing means disposed generallyparallel with said core between said mounting plate and said armture, asounding board for supporting said transducer, and means rigidlyconnecting said armature to said sounding board.

2. An electro-mechanical transducer comprising a driving elementincluding a magnetic core and a coil of wire wound on said core, saidcore being solidly attached to a mounting plate, an armature means ofmagnetizable material disposed in axially spaced relation with saidmagnetic core, resilient coupling means connecting said armature meanswith said mounting plate for permitting relative motion and mutual powertransfer in all directions between said core and said armature means,and permanent magnet disposed generally parallel with said core betweensaid mounting plate and said armature means.

3. A transducer element according to claim 1 or 2, wherein means isprovided for mounting the armature on the sounding board in partialspaced relation therewith, whereby the armature may vibrateindependently of the sounding board and its attachment thereto forimparting sound into the surrounding area.

4. An electro-mechanical transducer comprising a magnetic drivingelement and a magnetically driven element, means for resilientlycoupling said driven element and said driving element in spaced relationwith each other, a sounding board for supporting said transducer, andcoupling means extending from the driven element for solidly attachingsaid driven element in spaced relation with and to a said soundingboard, whereby said driven element may vibrate concurrently with andindependently of said sounding board.

5. An electro-mechanical transducer for producing sound vibrationcomprising a magnetic core and an electrical coil wound on said core,said magnetic core being fixedly attached to a mounting plate, amagnetizable armature axially spaced from said magnetic core, spacingmeans extending generally parallel with said core between said mountingplate and said armature, and means for adjusting the axial spacingbetween said magnetic core and said armature, said adjusting meanscomprising axially slanting cam tracks located at the end of saidspacing means opposite said armature, said mounting p-late havingrunners disposed to ride on said slanting cam tracks, and said mountingplate being rotatable on said tracks for moving said magnetic coreaxially toward and away from said aramture.

6. A transducer according to claim 5 wherein said spacin g meanscomprises a permanent magnet.

7. A transducer according to claim 5 or 6 wherein means are provided forholding said totatable mounting plate in a fixed position relative tothe armature.

8. An electro-mechanical transducer according to claim 2 or 6 whereinthe permanent magnet is a body having an opening extending therethroughfrom end to end, and said magnetic core extends into the opening of saidbody from the end thereof remote from .the armature.

9. With an electro-mechanical transducer according to any one of claims1, 2, 4, and 5 the combination of a dynamic speaker electrical coupledwith said transducer to a signal source common to both the transducerand dynamic speaker.

References Cited UNITED STATES PATENTS 2,946,995 7/1960 Gosswiller340-388 2,957,053 10/1960 Chichester 179-114 3,366,749 1/1968 Ries179-1155 KATHLEEN H. CLAFFY, Primary Examiner.

ARTHUR A. MCGILL, Assistant Examiner.

US. Cl. X.R. 179-101

